1. Field of the Invention
The present invention relates to a method and apparatus for treating dusts containing zinc and/or lead in the form of oxides to a retrievable form of their respective pure metals.
2. Description of Related Art
Various industries generate a number of kinds of dusts, some of which contain among other metals zinc and/or lead. For example, steelmaking dusts from electric furnaces usually contain oxides of zinc and/or lead as well as oxides of iron. More particularly, in the automotive industry where steelmaking materials thrown into the electric furnace are car shredder dusts, the dusts generated from the electric furnace will typically contain zinc from automotive steel sheets constructed of galvanized steel sheets, and the dusts will further typically contain lead from car fuel tanks constructed of lead-and-tin-coated steel plates.
In the case of press scraps generated during the manufacture of automobiles, which contain zinc in the form of its substantially pure metal, the zinc can be removed from the galvanized steel sheet scraps by heating the scraps under reduced pressure to cause the zinc to evaporate, as in Japanese Patent Publication No. HEI 4-346681, the disclosure of which is hereby incorporated by reference. However, zinc and/or lead when in the form of oxides rather than their substantially pure metals cannot be retrieved and recycled using the method of Japanese Patent Publication No. HEI 4-346681.
Among other known methods to remove zinc from dusts, there is a method using a rotary kiln where the dusts are heated using a burner and the zinc oxides contained in the dusts are reduced using coke or coal. However, since the dusts have to be heated to extremely high temperatures, this method is disadvantageous in that a large energy cost is necessary and the zinc in the dusts tends to be re-oxidized making it difficult to retrieve the zinc. There is another known method to remove zinc from dusts where the zinc is evaporated at a high temperature generated by a plasma and is retrieved in the form of its pure metal by a Pb splash condenser. However, this method is also disadvantageous in that it tends to cause environmental problems when the method is performed on-site.
Therefore, dusts containing zinc and/or lead are now typically treated by embedding them in the ground so as to satisfy regulations for waste disposal. However, the embedding treatment of dusts is problematic due to the lack of waste sites and the high cost associated with disposal. Further, since all the materials of the dusts are wasted without recycling, in the embedding treatment, the treatment method is not desirable from the viewpoint of material economy.